Heavy rainfall may lead to flooding and travel disruption, the Met Office has warned.
A yellow weather warning has been put in place by the Met Office from 6am on Wednesday, December 23 to 6am on Thursday 24.
The warning covers most of Pembrokeshire, except for Fishguard and areas to the north of the county.
It is unknown at this time where the heaviest rains will fall.
A statement by the Met Office said: "Spells of heavy rain are expected across southern England and parts of south Wales during Wednesday and into early Thursday.
"The heaviest rain is likely to develop during Wednesday evening and continue overnight before clearing away to the southeast by Thursday morning.
"Much of the area is likely to see 20-40 mm through this period.
"There is still some uncertainty in exactly where the heaviest rain will fall but there is the potential for some places to see as much as 50-70 mm of rain.
"The heavy rain will be accompanied by strong north to northeasterly winds during Wednesday evening and overnight."
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